1. Field of the Invention
This invention relates to array antennas and is particulary concerned with providing time delay steering to array antenna elements.
2. Description of the Prior Art
An array antenna consists of a group of antenna elements uniformly spaced apart to form an array. The array can be used for transmitting a beam of microwave energy in a chosen direction or receiving a microwave signal from a particular direction. This beam steering is achieved by controlling the relative timing or phasing of the individual elements.
The most common means of steering a beam in an antenna array is to control the relative phase of the signal at the elements. For the case of a flat antenna array, if all the elements are operated in unison, the beam will be pointed in the broadside direction, which is the direction perpendicular to the plane of the array. If a linearly increasing phase shift is introduced across the face of the array, the beam will be deflected at some angle from the broadside direction. Such antenna systems, referred to as phased arrays, are employed in applications where it is required to steer the beam rapidly in space and where the use of parabolic dish antennas is not practical.
Controlling the relative phase of each of the antenna elements requires that each element contain a phase shifting device and that an electronic control system be used to control the phase of each of the elements. However, the wide scale use of phased arrays has been limited by the high cost of their complex circuitry. Furthermore, if the phase shifting circuit is adjusted to steer in a particular direction, this setting will only be valid for a particular frequency. Adjacent frequencies will be transmitted or received with directional errors, a phenomena known as "squint". Therefore, known phase shifting techniques impose a limit on the frequency range of operation.
Another technique that is used to steer the beam in an array antenna is to control the relative timing of the transmitted or received signal at the array element. In the tansmission mode, if the signal at each of the elements is emitted in unison, a wavefront is formed that is parrallel to the plane of the array. The signal beam is directed perpendicular to the wavefront, therefore, when the signal is emitted from the antenna elements in unison, the beam is directed perpendicular to the plane of the array (the boresight direction). When the emission from the antenna elements is not in unison, but is varied in time along the array the angle of the wavefront relative to the plane of the array will change and the beam will be steered away from boresight. If, for example, the signal emission from any element relative to its nearest adjacent element is delayed a time t and each element is spaced a distance d apart, the steered angle .phi. between the boresight direction and the beam direction is given by the formula sin .phi.=t c/d, where c represents the velocity of electromagnetic propagation in space. True time delay techniques allow antenna arrays to operate over extremely wide frequency ranges as the delay techniques are frequency independent.
The use of fiberoptic communication systems is known A commercially available laser unit is used to convert a microwave signal to an amplitude modulated optical signal. The optical signal travels through the optical fiber to where it is converted back to a microwave signal by an optical detector and a microwave amplifier, which are commercially available. Optical techniques have been suggested to control array elements. Schemes have been proposed to use a selection of optical fibers with lengths arranged in a binary or quadratic sequence and to switch in a series string combination to achieve a desired timing. This would result in a very complex control employing thousands of optical fibers and optical switches for even the simplest array.
A scheme using two sets of fiber, each having a parabolic distribution of lengths has been described in U.S. Pat. Nos. 5,325,102 and 5347,288. By aligning these two sets of fibers and moving one set relative to the other, a linear and variable set of delay paths can be generated which can be incorporated into an antenna array to provide the timing needed to form and steer the beam. This scheme is difficult and costly to fabricate as it is necessary to align a large set of fiber optics end to end with a second set and to be able to reposition and realign each time the beam steering is adjusted.
A further drawback of the optical commutator approach to beam steering is that the signal is interrupted during the repositioning of the transmitted and received beams. This means the approach cannot be used for communications applications that require continuous and uninterrupted service.